Hydrogen (H2) has been produced in an amount of about five hundred billion Nm3 all over the world. The hydrogen has attracted much attention as future clean energy as well as has been applied for a variety of uses such as refinement of oil or production of ammonia. For example, a fuel cell is capable of efficiently supplying electricity when the hydrogen is supplied externally thereto. However, the hydrogen is highly reactive gas, so that it is difficult to be transported and stored. Therefore, there has been a need for a safe and inexpensive transportation and storage technology in order to stably supply the hydrogen. In the field of the fuel cell, there has been a problem that a poisoning substance is by-produced on a surface of an electrode catalyst by the action of carbon monoxide. Thus, there has been a need to supply high purity hydrogen generally containing 10 ppm or less of carbon monoxide.
As a hydrogen storage method, at present, a method for storing hydrogen as high pressure gas in a gas cylinder is commonly used. However, in this method, there are problems of safety upon transportation of the high pressure gas, and hydrogen brittleness of container materials. A method for storing hydrogen gas in the form of liquid hydrogen under an extremely low temperature is also used. However, there are problems that much energy is consumed in a liquefaction process and that the liquid hydrogen is lost in a percentage of 3% per day to 6% per day due to vaporization.
In order to solve the above described problems with regard to hydrogen transportation and storage technologies, there has been considered a method for storing hydrogen as liquid fuel (e.g., methanol and formic acid) which is obtained by hydrogenating carbon dioxide. For example, formic acid (HCOOH) has recently been attracted the attention as a hydrogen storage material since the formic acid, which is in the liquid form at normal temperature and has a relatively low toxicity, can be reversibly converted to hydrogen (H2) and carbon dioxide (CO2). However, there has been a problem that a dehydrogenation reaction of the formic acid using a conventionally known catalyst generally requires a high temperature of 200° C. or higher, and generates carbon monoxide as a by-product. Therefore, there has been a need to develop a catalyst which allows high quality hydrogen to be produced from the formic acid under a mild condition.
Recently, many reports have been made with regard to a dehydrogenation reaction of formic acid using a metal complex catalyst (PTLs 1 and 2 and NPLs 1 to 8). In these reactions, although hydrogen is produced through dehydrogenation of formic acid, carbon monoxide is hardly by-produced. However, most of them need organic solvents or amine additives. On the other hand, a reaction in water free of organic additives is problematic in low catalytic activity and durability (PTLs 3 to 7, NPLs 9 to 12). Besides the above reports, the present inventors have been found catalysts which are extremely highly active in the dehydrogenation reaction of formic acid in water free of organic additives. However, these catalysts are problematic in durability because they are easily decomposed in a high-concentration formic acid solution or under a high temperature reaction condition (PTLs 8 to 14, NPLs 13 to 21).